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R-select Algorithm.c
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R-select Algorithm.c
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#include<stdio.h>
#include<stdlib.h>
int partition(int arr[], int l, int r, int k);
int findMedian(int arr[], int n) ;
int kthSmallest3(int arr[], int l, int r, int k) ;
int kthSmallest5(int arr[], int l, int r, int k) ;
int kthSmallest7(int arr[], int l, int r, int k) ;
void swap(int *a, int *b) ;
void Sort(int arr[], int n) ;
int partition(int arr[], int l, int r, int x) ;
int main()
{
int k,n,g,i;
printf("Enter the number of elements, value for k and value for g\n");
scanf("%d %d",&n,&k);
printf("Enter the elements of array\n");
int* arr=(int *)malloc(sizeof(int)*n);
for(i=0;i<n;i++)
scanf("%d",&arr[i]);
printf("The kth smallest element using 3 groups is:%d\n", kthSmallest3(arr,0,n-1,k));
printf("The kth smallest element using 5 groups is:%d\n", kthSmallest5(arr,0,n-1,k));
printf("The kth smallest element using 7 groups is:%d\n", kthSmallest7(arr,0,n-1,k));
return 0;
}
int findMedian(int arr[], int n)
{
Sort(arr, n);
return arr[n/2];
}
int kthSmallest5(int arr[], int l, int r, int k)
{
// If k is smaller than number of elements in array
if (k > 0 && k <= r - l + 1)
{
int n = r-l+1; // Number of elements in arr[l..r]
// Divide arr[] in groups of size 5, calculate median
// of every group and store it in median[] array.
int i, median[(n+4)/5]; // There will be floor((n+4)/5) groups;
for (i=0; i<n/5; i++)
median[i] = findMedian(arr+l+i*5, 5);
if (i*5 < n) //For last group with less than 5 elements
{
median[i] = findMedian(arr+l+i*5, n%5);
i++;
}
// Find median of all medians using recursive call.
// If median[] has only one element, then no need
// of recursive call
int medOfMed = (i == 1)? median[i-1]:
kthSmallest5(median, 0, i-1, i/2);
// Partition the array around a random element and
// get position of pivot element in sorted array
int pos = partition(arr, l, r, medOfMed);
// If position is same as k
if (pos-l == k-1)
return arr[pos];
if (pos-l > k-1) // If position is more, recur for left
return kthSmallest5(arr, l, pos-1, k);
// Else recur for right subarray
return kthSmallest5(arr, pos+1, r, k-pos+l-1);
}
// If k is more than number of elements in array
return 9999;
}
int kthSmallest7(int arr[], int l, int r, int k)
{
// If k is smaller than number of elements in array
if (k > 0 && k <= r - l + 1)
{
int n = r-l+1; // Number of elements in arr[l..r]
// Divide arr[] in groups of size 7, calculate median
// of every group and store it in median[] array.
int i, median[(n+6)/7]; // There will be floor((n+6)/7) groups;
for (i=0; i<n/7; i++)
median[i] = findMedian(arr+l+i*7, 7);
if (i*7 < n) //For last group with less than 7 elements
{
median[i] = findMedian(arr+l+i*7, n%7);
i++;
}
// Find median of all medians using recursive call.
// If median[] has only one element, then no need
// of recursive call
int medOfMed = (i == 1)? median[i-1]:
kthSmallest7(median, 0, i-1, i/2);
// Partition the array around a random element and
// get position of pivot element in sorted array
int pos = partition(arr, l, r, medOfMed);
// If position is same as k
if (pos-l == k-1)
return arr[pos];
if (pos-l > k-1) // If position is more, recur for left
return kthSmallest7(arr, l, pos-1, k);
// Else recur for right subarray
return kthSmallest7(arr, pos+1, r, k-pos+l-1);
}
// If k is more than number of elements in array
return 9999;
}
int kthSmallest3(int arr[], int l, int r, int k)
{
// If k is smaller than number of elements in array
if (k > 0 && k <= r - l + 1)
{
int n = r-l+1; // Number of elements in arr[l..r]
// Divide arr[] in groups of size 3, calculate median
// of every group and store it in median[] array.
int i, median[(n+2)/3]; // There will be floor((n+2)/3) groups;
for (i=0; i<n/3; i++)
median[i] = findMedian(arr+l+i*3, 3);
if (i*3 < n) //For last group with less than 3 elements
{
median[i] = findMedian(arr+l+i*3, n%3);
i++;
}
// Find median of all medians using recursive call.
// If median[] has only one element, then no need
// of recursive call
int medOfMed = (i == 1)? median[i-1]:
kthSmallest3(median, 0, i-1, i/2);
// Partition the array around a random element and
// get position of pivot element in sorted array
int pos = partition(arr, l, r, medOfMed);
// If position is same as k
if (pos-l == k-1)
return arr[pos];
if (pos-l > k-1) // If position is more, recur for left
return kthSmallest3(arr, l, pos-1, k);
// Else recur for right subarray
return kthSmallest3(arr, pos+1, r, k-pos+l-1);
}
// If k is more than number of elements in array
return 9999;
}
void swap(int *a, int *b)
{
int temp = *a;
*a = *b;
*b = temp;
}
void Sort(int arr[], int n)
{
int i, j, min_idx;
for (i = 0; i < n-1; i++)
{
min_idx = i;
for (j = i+1; j < n; j++)
if (arr[j] < arr[min_idx])
min_idx = j;
swap(&arr[min_idx], &arr[i]);
}
}
int partition(int arr[], int l, int r, int x)
{
int i,j;
for (i=l; i<r; i++)
if (arr[i] == x)
break;
swap(&arr[i], &arr[r]);
i = l;
for (j = l; j <= r - 1; j++)
{
if (arr[j] <= x)
{
swap(&arr[i], &arr[j]);
i++;
}
}
swap(&arr[i], &arr[r]);
return i;
}